Eusiroides dimorphus Guedes-Silva, Souza-Filho and Tavares, 2024

Guedes-Silva, Elkênita, Souza-Filho, Jesser F. & Tavares, Marcos, 2024, Three new species of Amphipoda (Crustacea: Peracarida) from the remote oceanic island of Trindade, off the coast of south-eastern Brazil, Journal of Natural History 58 (45 - 48), pp. 2075-2098 : 2087-2094

Eusiroides dimorphus Guedes-Silva, Souza-Filho and Tavares sp. n.

( Figures 10–13 )

Type material

Holotype: male (5.4 mm), MZUSP 45151 , PROTRINDADE – Trindade Island , off Espírito Santo, Brazil. Praia do Noroeste , in calcareous algae, diurnal scuba diving, 9.5 m depth, 20.496°S, 29.343°W. J.B. Mendonça col., 06 August 2018 . Paratypes: 1 male (2.7 mm), MZUSP 45602 , PROTRINDADE – Trindade Island , off Espírito Santo, Brazil . Praia do Noroeste , in calcareous algae, diurnal scuba diving, 9.5 m depth, 20.496°S, 29.343°W. J. B. Mendonça col., 06 August 2018 . 1 female, MZUSP 45608 , ibidem, 06 August 2018 . 1 male (dissected and drawn, 1 slide), MZUSP 45152 , 2 males, 1 female, MZUSP 45613 , ibidem, SECON ( Enseada dos Portugueses ), diurnal scuba diving, 12.2 m depth, 20.506°S, 29.312° W. J.B. Mendonça col., 18 July 2013 . 1 female (4.62 mm), MZUSP 45603 , ibidem, 10.2 m depth, 20.506°S, 29.312°W. J.B. Mendonça col., 22 November 2014 . 1 male, MZUSP 45609 , ibidem, Ponta da Calheta , artificial reef, 16.7 m depth, 20.505°S, 29.309°W. J.B. Mendonça col., 24 May 2022 . 1 female, MZUSP 45616 , ibidem, Ponta da Calheta , diurnal scuba diving, 9.9 m depth, 20.505°S, 29.309°W. J.B. Mendonça col., 26 October 2014 . 1 male, MZUSP 45617 , ibidem, 17.7 m depth, 20.505°S, 29.309°W. J.B. Mendonça col., 03 August 2015 . 1 female, MZUSP 45610 , ibidem, Parcel/Tartarugas , diurnal scuba diving, 9.5 m depth, 20.517°S, 29.299°W. J.B. Mendonça col., 26 June 2012 . 2 female, MZUSP 45611 , ibidem, Enseada da Cachoeira , diurnal scuba diving, 13.8 m depth, 20.516°S, 29.338°W. J.B. Mendonça col., 09 July 2012 . 1 unsexed, MZUSP 45612 , ibidem, Ponta do Monumento , diurnal scuba diving, 8.1 m depth, 20.503°S, 29.343°W. J.B. Mendonça col., 13 July 2012 . 1 male, MZUSP 45615 , ibidem, Praia dos Portugueses (Rampa nova), diurnal scuba diving, 10.7 m depth, 20.505°S, 29.316°W. J.B. Mendonça col., 23 April 2014 . 1 female, MZUSP 45618 , ibidem, Ponta Norte , diurnal scuba diving, 10.9 m depth, 20.489°S, 29.338°W. J.B. Mendonça col., 23 July 2018 . 1 unsexed, MOUFPE 22043 , ibidem, Ponta Norte , diurnal scuba diving, 15.6 m depth, 20.489°S, 29.338°W. J.B. Mendonça col., 03 July 2012 . 1 unsexed, MOUFPE 22044 , ibidem, Ponta Norte , scuba diving, 14 m depth, 20.489°S, 29.338°W. J.B. Mendonça col., 23 October 2014 .

Diagnosis

Antennae with stout peduncular articles, with calceoli on flagellum. Antenna 1 accessory flagellum 1-articulate, shorter than first proximal article of flagellum. Coxae 1–3 with short spines on posteroventral corner. Gnathopod 1 smaller than gnathopod 2, propodus with palm outer margin dentate bearing eight robust setae and one robust seta defining palmar corner, outer margin dentate and inner margin finely serrate. Epimera 1–2 with one short acute spine on posteroventral corner. Epimeron 3 posterior margin not serrate, with two short curved and acute spines on posteroventral corner. Uropod 2 outer ramus short, 3/4 of inner ramus length. Telson deeply cleft (50%), lobes differ in size, left lobe longer than right.

Description

Head

Head with well developed, reniform eyes meeting dorsally. Antenna 1 shorter than body length (0.5×); peduncle article 1 longer than article 2 (1.6×); article 3 shorter than article 2 (0.5×); accessory flagellum minute, 1-articulate, shorter than flagellum article 1; flagellum 2.3× longer than peduncle calceoli, and aesthetascs present. Antenna 2 shorter than antenna 1 (0.6×); peduncular articles setose, calceoli present. Lower lip inner lobe short; outer lobes setose subapically and on inner margin. Mandible molar triturative, well developed; incisor and lacinia mobilis dentate; accessory setal row with three serrated setae; palp 3-articulate, article 2 with six slender setae, article 3 minutely setose, bearing row of marginal slender setae and four apical slender setae. Maxilla 1 inner plate with five short, slender setae distally; outer plate with nine robust setae, being five dentate; palp 2-articulate with eight long setae. Maxilla 2 inner plate longer than outer plate. Maxilliped inner plate short, apical margin with three robust setae; outer plate 1/2 of palp length, inner margin and apex with row of long setae; palp 4-articulate, inner margin setose, article 4 without unguis.

Pereon

Gnathopod 1 coxa anterior margin produced, posteroventral margin with three short spines; basis 3.1× longer than wide, anterior margin with tuft of setae at anterodistal angle; ischium subquadrate, as long as wide, posterior margin with tuft of setae; merus subtriangular, about 1.7× longer than wide, posterior margin heavily setose; carpus lobate, lobe heavily setose; propodus oval, 1.9× longer than carpus, palm acute, densely setose, outer margin with seven large, blunt spines, each with one stout seta, inner margin minutely serrate; dactylus falcate. Gnathopod 2 coxa subrectangular, 1.5× longer than wide, posterior margin with stout seta; posteroventral margin with two short spines, inner surface with an oblique row of four slender setae; basis 3.7× longer than wide, posterior margin with a long setae, anterior margin with tuft of setae at anterodistal angle; ischium subquadrate, as long as wide; merus subtriangular, about 1.8× longer than wide, posterodistal angle acute; carpus lobate, lobe heavily setose; propodus oval, 2.8× longer than carpus, palm acute, inner face densely setose, outer margin with six large, blunt spines each with a stout setae, inner margin minutely serrate, palmar corner defined by a stout seta; dactylus falcate. Pereopod 3 coxa subrectangular, 1.5× longer than wide, posterior margin with a stout seta, posteroventral margin with one short spine; basis slender, 3.4× longer than wide; ischium short, 1.3× longer than wide; merus 0.5× shorter than basis, anterior margin slightly produced; carpus shorter than propodus (0.7×), posterior margin with two robust setae; propodus subrectangular, 3.8× longer than wide, posterior margin with five robust setae and one posterodistal locking seta; dactylus falcate, unguis present. Pereopod 4 coxa wide, 1.2× longer than wide, posterior margin produced into a subacute lobe; basis slender, 3.3× longer than wide; ischium short, 1.3× longer than wide; merus 0.6× shorter than basis, anterior margin slightly produced; carpus shorter than propodus (0.7×), posterior margin with two robust setae; propodus subrectangular, 4.5× longer than wide, posterior margin with five robust setae and one posterodistal locking seta; dactylus falcate, unguis present. Pereopod 5 coxa bilobed, lobes subequal; basis ovate, 1.4× longer than wide, anterior margin with row of robust setae, posterior margin with proximal half finely serrate, posterodistal angle produced; ischium short, subrectangular; merus subtriangular, posterodistal angle produced; carpus to dactylus lost. Pereopod 6, coxa weakly bilobed; basis ovate, 1.4× longer than wide, anterior margin with row of robust setae, posterior margin finely serrate; ischium short, subrectangular; merus subtriangular, posterodistal angle produced, 0.8× of propodus length; carpus slender 4.5× longer than wide, anterior margin with row four robust setae and two locking setae subdistally; dactylus falcate, unguis present. Pereopod 7 coxa ovate; basis ovate, 1.4× longer than wide, anterior margin with a row of robust setae, posterior margin finely serrate; ischium short and subrectangular; merus subtriangular, posterodistal angle produced, 0.8× of propodus length; carpus slender 4.7× longer than wide, anterior margin with a row six robust setae and two locking setae subdistally; dactylus falcate, unguis present.

Pleon

Epimeron 1 surface with oblique ridge, anteroventral margin with five slender setae; posteroventral angle produced into an acute spine; epimeron 2 surface with oblique ridge, one robust seta on ventral margin medially, posteroventral angle produced into an acute spine; epimeron 3 posterior margin with two spines on posteroventral angle. Uropod 1 peduncle inner margin with two robust setae, inner distal angle with a stout seta, outer margin with five robust setae; outer ramus 0.8× inner ramus length, with two marginal robust setae and two apical robust setae; inner ramus with three marginal robust setae and five apical robust setae. Uropod 2 peduncle inner margin with a robust setae, inner distal angle with a robust seta, outer margin naked, outer distal angle with a robust seta; outer ramus 0.6× inner ramus length, with two marginal robust setae and four apical robust setae; inner ramus with four marginal robust setae and four apical robust setae. Uropod 3 lanceolate and foliaceous, peduncle subrectangular, about 2× longer than wide, inner distal angle with one robust seta; inner ramus as long as outer ramus, inner margin with four robust setae, outer margin with three robust setae; outer ramus, inner margin with six robust setae and three plumose setae. Telson cleft (about 1/2 length) lateral margin with two short, plumose setae; lobes unequal in length, right lobe with one slender seta and one plumose seta medially, apical margin with a short, simple seta; left lobe shortened, about 0.8× of right lobe length, without setae, apically rounded.

Female

Sexual dimorphism weak.

Etymology

The specific epithet is derived from the Latin ‘dimorphus’ (= occurring in or representing two distinct forms) in allusion to the lobes of telson unequal in length.

Type locality

Praia do Noroeste, 20.496°S, 29.343°W, 9.5 m, Trindade Island, off Espírito Santo, Brazil.

Variations

Several species in Eusiroides were described on the basis of only one or two specimens, so variation within species needs to be better documented. In Eusiroides dimorphus sp. n., the lobes of the telson are typically unequal in length, with the left lobe naked apically and distinctly shorter than the right lobe, which is provided with one apical seta ( Figure 13 , T (a)). However, in the males MZUSP 45602 (2.7 mm) and MZUSP 45152 (2.3 mm) the left lobe is only a little shorter than the right lobe ( Figure 13 , T (b,c)) and the apical seta can be lacking ( Figure 13 , T (c)), but there is no indication of the presence of setal insertion (socket). In the female MZUSP 45613 both lobes are provided with an apical seta and the left and right lobes are almost equally developed, with the left lobe minutely shorter than the right one ( Figure 13 , T (d)). Also, the submarginal setation of lobes can vary slightly between specimens ( Figure 13 , T (a-d)). Also, the posterior margin of epimeron 3 is usually provided with two spines in most specimens although three to four spines can also occur; one female (4.62 mm, MZUSP 45603) has three spines on the right side and four on the left side and another female (5.74 mm, MZUSP 45609) has two spines on the right side and three on the left side.

Remarks

The species of Eusiroides can be divided into four groups according to the morphology of the posterior margin of epimeron 3, as follows:

Group 1 – Epimeron 3 with posterior margin strongly serrate: E. caesaris Stebbing, 1888 , E. sarsi, Chevreux, 1900 , E. japonica Hirayama, 1985 , E. lippus ( Haswell, 1879) , E. monoculoides ( Haswell, 1879) , E. pilopalpus Jung, Kim and Yoon, 2016 , and E. yucatanensis McKinney, 1980 .

Group 2 – Epimeron 3 with posterior margin smooth, posteroventral angle without a spine – E. crassi Stebbing, 1888 , E. picta ( Giles, 1890) , and E. stenopleura Barnard KH, 1932 .

Group 3 – Epimeron 3 with posterior margin smooth, posteroventral angle with a spine – E. dentimera Ledoyer, 1982 , E. georgiana Barnard KH, 1932 (1), E. lucai Siqueira and Iwasa –Arai, in Iwasa–Arai et al. 2021.

Group 4 – Epimeron 3 posterior margin armed with 2–4 spines: E. diplonyx Walker, 1909 (3 spines), E. dimorphus sp. n. (2–4 spines), E. orchomenipes Walker, 1904 (3 spines).

The poorly known species E. dellavallei Chevreux, 1899 , cannot be placed with certainty in any of the above four groups.

Eusiroides dimorphus sp. n. can be distinguished from all the other species in group 4 by the possession of 2–4 spines on the posterior margin of epimeron 3, including a spine on the posteroventral angle, whereas the remaining two species in group 4 have three spines on posterior margin of epimeron 3 and posteroventral angle subrectangular, lacking spine. Eusiroides dimorphus sp. n. additionally differs from E. lucai , a species also known from Trindade Island but not caught during our samplings, as follows (characters of E. lucai within brackets): palms of gnathopods 1–2 outer margin strongly dentate (vs not dentate), outer ramus of uropod 2 reaching 2/3 of the inner ramus length (vs reaching 1/2 of inner ramus length), and telson lateral margins with only a few short setae (vs lined with short setae).

Figures

Figure 10. Eusiroides sp. n. Habitus, holotype, male (5.4 mm), Trindade Island, off Espírito Santo, Brazil, 9.5 m depth, 20.496°S, 29.343°W, 08 August 2018, MZUSP 45151. Head and mouthparts, paratype male (2.3 mm), Trindade Island, off Espírito Santo, Brazil, 12.2 m depth, 20.506°S, 29.312°W, 18 July 2013,MZUSP 45152. Scale bars: habitus = 1 mm; other parts = 0.1 mm. Abbreviations: Hb,habitus;Md, mandible; Mx, maxilla; LL, lower lip; Mxp, maxilliped; Hd, head.

Figure 11. Eusiroides sp. n. Paratype male (2.3 mm), Trindade Island, off Espírito Santo, Brazil, 12.2 m depth, 20.506°S, 29.312°W, 18 July 2013, MZUSP 45152. Scale bars = 0.1 mm. Abbreviations: Gn, gnathopod; P, pereopod.

Figure 12. Eusiroides sp. n. Paratype male (2.3 mm), Trindade Island, off Espírito Santo, Brazil, 12.2 m depth, 20.506°S, 29.312°W, 18 July 2013, MZUSP 45152. Scale bars = 0.1 mm. Abbreviations: Ep, epimeron; P, pereopod.

Figure 13. Eusiroides sp. n. Holotype: male telson (T(a), 5.4 mm), Trindade Island, off Espírito Santo, Brazil, 9.5 m depth, 20.496°S, 29.343°W, 08 August 2018, MZUSP 45151. Paratypes: male telson (T(b), 2.7 mm), MZUSP 45602, same as holotype. Telson, male (T(c), 2.3 mm) and female (T(d), 4.6 mm), Trindade Island, off Espírito Santo, Brazil, 12.2 m depth, 20.506°S, 29.312°W, 18 July 2013, MZUSP 45152. Scale bars = 0.1 mm. Abbreviations: T, telson; Ur, uropod.